This invention relates to an error correcting system which comprises an encoder and a decoder to transmit and receive a sequence of error correcting codewords through a transmission path, such as a radio channel.
In an error correcting system of the type described, error correction has been performed in various manners so as to correct a single error, a double error, a triple error, a t-tuple error, or the like which might occur during transmission. At any rate, redundancy bit or bits are added as a redundancy signal in an encoder to each information signal unit (namely, input signal unit) which is composed of a predetermined bit number of information bits. A combination of the information bits and the redundancy bits is sent as an error correcting codeword from the encoder to a decoder. In the decoder, the error correcting codeword is decoded into the information bits with error or errors corrected by the use of the redundancy bits.
It is well known in the art that an increase of the redundancy bits brings about an increase of bits which can be corrected by the error correcting codewords. In other words, error correction is given a high ability with an increase of the redundancy bits. The ability of error correction may be referred to as an error correction ability. However, an increase of the redundancy bits results in a reduction of a transmission rate which is determined by an allowable frequency band of a transmission path. Therefore, the bit number of the redundancy bits should be selected in consideration of the allowable frequency band of the transmission path and the information bits included in each information signal unit.
The bit number of the information bits discretely and extensively increases as the number of the redundancy bits increases one by one. Such a discrete increase of the information bits makes it difficult to exactly adjust the error correcting codeword to the allowable frequency band. Under the circumstances, the error correcting codeword has a transmission rate which is very lower than an allowable transmission rate allowed by the allowable frequency band. This shows that an unused frequency band remains in the allowable frequency band at the sacrifice of the error correcting ability. In other words, neither the allowable frequency nor the error correcting ability of the error correcting codeword is effectively used in the conventional error correcting system.